Low Temperature Co-fired Ceramic (LTCC) technology is an electronic packaging platform especially suitable for high frequency system level packaging applications. A typical LTCC circuit substrate is formed by laminating multiple layers of ceramic tape under pressure and then firing the tape at high temperatures in the range of 800 to 900 degrees Celsius. On firing, LTCC forms a monolithic circuit containing electrical interconnections and provides for a highly reliable integrated circuit chip carrier platform. Electrical interconnections on LTCC substrates are generally formed by using thick film metallizations of gold, silver, or copper metals. Being a ceramic material, LTCC is a high reliability system and also has very good thermal properties in addition to extremely low dielectric loss for electrical signals. LTCC has a coefficient of thermal expansion (CTE) relatively close to that of semiconductor materials used for fabricating chips thereby making high reliability flip chip attachment possible.
Fabrication of microwave/millimeterwave circuits such as filters, amplifiers, oscillators etc. require very closely spaced conductor traces (line width and spacing of the order of 1 to 2 mil). The current state of the art process for thick film metal patterning on the internal layers of LTCC is screen printing, which is an additive process. LTCC technology using screen printing is limited to 3 mil line width and line spacing in the best case and hence will not be sufficient for efficient fabrication of microwave and millimeter wave circuits (circuits which operates above a frequency of 30 GHz). Other technologies such as vacuum deposition, electroplating etc. which can be used on the exterior surfaces of LTCC circuits cannot be used on the interior layers since patterning of internal layers is done while the LTCC tape is still in an unfired state.
The problem to be solved is to successfully manufacture a Sievenpiper EBG structure in LTCC packages for applications at millimeterwave frequencies. The difficulty is found in fabricating narrow gaps of less than 3 mils (75 um) between isolated conductive patches which are internal to a fired LTCC package. It is well known that patch antenna elements launch power into TM surface wave modes in their E-plane, and they are capable of launching TE surface wave modes in their H plane provided the host substrate is sufficiently thick to support the fundamental TE mode. These parasitic surface waves degrade antenna performance by diffracting at substrate edges where the edges become secondary sources of radiation. The net result is a loss of broadside directivity, higher side lobe levels, potentially multiple main beams, higher cross polarization, and poor front-to-back ratio. At millimeterwave (MMW) frequencies, the excitation of parasitic surface waves becomes a serious issue for LTCC antennas.
Almost all of the EBG related publications whose application frequency is below 15 GHz reference manufacturing by conventional printed circuit board techniques where etching of a Cu clad laminate is employed. It is difficult and costly to etch gaps below 3 mils with printed circuit board technology, and therefore EBG structures fabricated in this manner are limited in frequency to below approximately 15 GHz.
LTCC has been long recognized as a potential and desirable substrate for fabrication of EBG structures. Some fabricated examples of LTCC EBG structures have been published with predicted bandgap frequencies as high as 50 GHz. Examples of EBG structures fabricated in LTCC are simply not found above 40 GHz because gap dimensions of 3 mils (75 um) or less are required. Previously, this range of gap dimension has not been manufacturable using standard LTCC materials and processes.
There exists a need to integrate Sievenpiper EBG structures into antenna packages for applications above 40 GHz, namely for 60 GHz WLAN, for 60 GHz backhaul point-to-point data links, for 77 GHz automotive radar, and for 94 GHz imaging radars. The problem has been that there was no viable manufacturing technique to fabricate EBG structures inside LTCC packages for use at millimeterwave frequencies above 40 GHz. A need exists to fabricate gaps in conductors narrower than 3 mils (75 um) as an enabling technology required to support commercial LTCC packaging applications in the 60 GHz to 100 GHz frequency range. A high demand in the market exists for high frequency/high speed packaging materials, such as GreenTape™ LTCC, which is a key materials platform in the microwave/milimeterwave market. The ability to fabricate antenna structures transmitting with high radiation efficiency as provided by structures and methods of the present inventions is critical for that industry.